Latest strike leaves the United States with no good way to exit Afghanistan.

The vicious, ISIL-style attack by the Pakistani Taliban on the Bacha Khan University campus near Peshawar has suddenly laid bare one of the nasty, unsung undercurrents of America’s withdrawal from Afghanistan, barely 25 miles to the west.

Quite simply, the United States has no path to an exit. Those forces of evil with whom we had been hoping to negotiate pale by comparison with their even nastier Taliban cousins further back behind the mountains of the rugged Hindu Kush. For years, America held firm to the hope that we had found elements of the Taliban with whom we could negotiate. But that has been exposed as the ultimate pipe dream. Perhaps they do indeed exist. But they don’t count for much when it comes to a search for peace, or at least an end to war, in that part of the world. It’s increasingly clear that the Taliban is not going to facilitate any graceful U.S. exit.

It had been a long-standing diplomatic tenet that Pakistan, through its Inter-Services Intelligence spy agency, had close ties with the Taliban. The regime in Islamabad saw that as a critical counterweight to the U.S.-backed government in Kabul, which in turn maintained close ties to India. India and Pakistan, each with formidable nuclear arsenals, have viewed each other as mortal enemies. The theory went that if we could get the Pakistanis on the side of peace and coerce their Taliban friends to the bargaining table, the combined weight of the world could persuade the Taliban to stand down.

Now all these ideas have been smashed to smithereens by a series of attacks on Pakistan by at least some elements of the Taliban. Born in Afghanistan, but hosted for years in the mountainous terrain of Pakistan’s North West Frontier province, many Taliban leaders and their followers now want nothing short of imposing a harsh sharia law on the entire nation of Pakistan.

Little more than a week before the campus attack that left at least 21 dead, diplomats from Pakistan, Afghanistan, China (which shares a border with both countries) and the United States met in Islamabad to discuss how best to revive talks between the Taliban and the Afghan government.

Such a revival hardly appears likely at this point.

The last time everyone convened was last July in Pakistan. It was subsequently revealed that Taliban leader Mullah Mohammad Omar, who supposedly had approved the talks, had been dead for two years. At that point, no one knew who might come to the bargaining table, or what they’d be able to guarantee even if any agreement could be reached. Omar’s putative successor, his deputy Mullah Akhtar Mansoor, might himself have been fatally shot last month in a battle with a rival, breakaway faction.

Moreover, for the moment, the Taliban writ large has the upper hand on both sides of the Afghan-Pakistan frontier. Last month, a dozen Taliban fighters killed dozens of adults and children outside the airfield in Kandahar, Afghanistan’s second largest city. There has been a string of similar terror attacks, many aimed at showing that even in the less hospitable rainy season, the Taliban is still a potent force.

America still has a major stake in all of this. First, there is growing evidence that the Islamic State terrorist group has taken root in some scattered stretches of Afghanistan’s Nangarhar province bordering Pakistan. That could be a game-changer. ISIL is even more ferocious than the Taliban, and it's more attractive to any number of Afghan warriors — particularly if the Taliban is seen to be negotiating with the officials of their great foe, the United States.

Moreover, the last time the Taliban ruled Afghanistan, its leaders gave carte blanche to al-Qaeda and Osama bin Laden, who planned the 9/11 attacks from the territory controlled by the Taliban.

So what is the United States to do?

We have little choice but to wait it out. Our forces must stay in some numbers on the ground for quite an indefinite future. There must also be at least the perception of a unified American position rather than a deeply divided United States slogging through a bitter political campaign for another year. The risk, otherwise, is another debacle along the lines of what happened when U.S. forces bolted from Iraq. Remember how that vacuum was filled. By ISIL.